When the narcissist is angry, unhappy, disappointed, injured, or hurt - he feels unable to express his emotions sincerely and openly since to do so would be to admit his frailty, his neediness, and his weaknesses. He deplores his own humanity - his emotions, his vulnerability, his susceptibility, his gullibility, his inadequacies, and his failures. So, he makes use of other people to express his pain and his frustration, his pent up anger and his aggression. He achieves this by mentally torturing other people to the point of madness, by driving them to violence, by reducing them to scar tissue in search of outlet, closure, and, sometimes, revenge. He forces people to lose their own character traits - and adopt his own instead. In reaction to his constant and well-targeted abuse, they become abusive, vengeful, ruthless, lacking empathy, obsessed, and aggressive. They mirror him faithfully and thus relieve him of the need to express himself directly.
Having constructed this writhing hall of human mirrors, the narcissist withdraws. The goal achieved, he lets go. As opposed to the sadist, he is no in it, indefinitely, for the pleasure of it. He abuses and traumatizes, humiliates and abandons, discards and ignores, insults and provokes - only for the purpose of purging his inner demons. By possessing others, he purifies himself, cathartically, and exorcises his demented self.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html